It is oftentimes desirable to coat the surface of a substrate. For example, structural components and engine parts of modern aircraft are commonly coated. In particular, the landing gear and various engine parts of modern aircraft can be coated, such as with a metallic coating, in order to improve the corrosion resistance and wear properties of the coated part and to permit the coated part to withstand relatively large fluctuations in temperature without performance degradation or structural weakening.
Substrates, such as the surfaces of various aircraft parts, are conventionally electrochemically coated with metallic coatings by immersing the substrates in aqueous solutions of various metals, such as nickel, chromium or cadmium. By directing an electrical current through the aqueous solution, metal particulates can be deposited from the aqueous solution onto the surface of the substrate, thereby coating the substrate with a metallic coating. However, due to the potentially harmful effects of aqueous solutions of various metals on the environment, including aqueous solutions of metals, such as nickel, chromium and cadmium, which are employed for electrochemically coating substrates, the electrochemical deposition of metals from an aqueous solution has been significantly restricted by various governmental guidelines and regulations. See, for example, the federal Clean Water Act.
Accordingly, several alternative methods of coating substrates have been developed. For example, coatings have been deposited by chemical vapor deposition methods. As known to those skilled in the art, however, the substrates which are to be coated by chemical vapor deposition must generally be heated to a relatively high temperature, typically ranging from several hundred to 1000.degree. C., to insure that the coating will sufficiently adhere to the substrate. For a number of substrates including metallic substrates, such as steel substrates and, more particularly, tempered steel substrates, which are sensitive to temperature fluctuations, the heating of the substrate to relatively high temperatures can be harmful. For example, the properties of a tempered steel substrate can be irreparably altered or damaged by exposure to the relatively high temperatures required during a chemical vapor deposition process.
In addition, several methods of melting metallic particles with a laser to form a coating on a metallic substrate have been developed. See U.S. Pat. No. 5,122,632 which issued Jun. 16, 1992 to Konrad Kinkelin; U.S. Pat. No. 5,043,548 which issued Aug. 27, 1991 to Erik J. Whitney et al. and which is assigned to General Electric Company; and U.S. Pat. No. 4,300,474 which issued Nov. 17, 1981 to Livsey and is assigned to Rolls-Royce Limited (hereinafter the "Livsey No. '474 patent"). For example, the Livsey No. '474 patent discloses an apparatus for applying metallic coatings to metallic substrates. In particular, the apparatus of the Livsey No. '474 patent simultaneously directs both a laser beam and a gas stream containing entrained particles of a metallic coating material toward a substrate. Due to the power delivered by the laser beam, the metallic particles are melted to form a molten pool of the metallic coating material upon the substrate. Consequently, upon solidification of the molten pool of the metallic coating material, a metallic coating is formed on the substrate.
As specified by the Livsey No. '474 patent, the gas stream containing the entrained particles of the metallic coating material is directed toward the substrate through an inclined tube. Due to the orientation of the inclined tube, the gas stream containing the entrained particles of the metallic coating material is directed both downwardly and laterally toward the surface of the substrate such that the momentum of the gas stream carries the entrained particles of the metallic coating material toward the substrate. In addition, the inclined tube of the apparatus of the Livsey No. '474 patent is oriented such that the gas stream is directed to the same portion of the substrate onto which the laser beam is focused such that the metallic coating particles are melted. However, since the metallic coating particles and the laser beam must be simultaneously directed to the same area of the substrate in order to properly melt the metallic coating particles, the position and orientation of both the laser beam and the inclined tube must be precisely controlled.